This invention relates to a differential capacitance probe. More particularly, the invention relates to a differential capacitance probe that detects small differences in complex dielectric constant of a process fluid as compared to a reference fluid. This differential capacitance probe yields a control error signal in real time such that the process can be controlled.
A problem that is frequently encountered in certain industrial processes is how to measure very small differences in complex dielectric constant where such differences are indicative of a critical parameter of a process that is to be controlled The relative dielectric constant, xcex5r, of water is approximately 80, and many critical aqueous processes will vary the xcex5r of water by a small fraction. Direct measurement of a small change in xcex5r, in the presence of other parameter sensitivities such as temperature coefficients, is very difficult. For many process control applications, however, the direct measurement of xcex5r is not necessary. What is needed is an error signal that indicates to a process controller that the process xcex5r is either above or below the desired xcex5r and, as corrections are made, that the sign and magnitude of the error signal measures the quality of the correction.
In view of the foregoing, it will be appreciated that providing a differential capacitance probe that provides an error signal in real time, thus enabling correction of a critical parameter of the process, would be a significant advancement in the art.
It is an object of the present invention to provide a differential capacitance probe that enables real time control of process operations involving aqueous solutions of organics wherein the process variable is a very small percent by weight of the solution.
It is also an object of the invention to provide a differential capacitance probe having a reference probe and a process probe both operated at the same temperature.
It is another object of the invention to provide a differential capacitance probe that yields an output that is a control error signal that can be used to control the process.
It is still another object of the invention to provide a differential capacitance probe that uses a pair of matched capacitance probes, one immersed in a sealed container of a reference fluid, and the other immersed in the process fluid.
It is yet another object of the invention to provide a differential capacitance probe that is suitable for use in industries such as food processing and paper mill operations.
It is a further object of the invention to provide a differential capacitance probe that does not generate hazardous waste.
These and other objects can be addressed by providing a system for controlling a process including a process aqueous dielectric fluid having at least one variable parameter, the system comprising:
(a) a first probe configured for being immersed in the process aqueous dielectric fluid, the first probe configured for measuring at least the capacitance of the process aqueous dielectric fluid; and
(b) a second probe configured for being immersed in a reference dielectric fluid wherein the reference dielectric fluid is isolated from the process aqueous dielectric fluid, the second probe configured for measuring at least the capacitance of the reference aqueous dielectric fluid;
(c) a power source, the first probe and the second probe being connected to the power source such that the current provided to the first probe and the second probe is directly proportional and the first probe generates a first measurement signal and the second probe generates a second measurement signal;
(d) means for detecting the difference between the first measurement signal and the second measurement signal and outputting an error signal, the error signal being used to promotionally alter the variable parameter to modify the process aqueous dielectric fluid.
In a preferred embodiment of this system, the second probe and reference dielectric fluid are sealed in a temperature-permeable container. Preferably, the first probe and the second probe are coupled in-parallel to the power source and the first probe and the second probe each comprise spaced apart plates having the same surface areas and distance between the plates. It is also preferred that the first measurement signal and the second measurement signal comprise current signals, and that the means for detecting differences between the current signals comprises an amplifier.
Another preferred embodiment of the invention relates to an apparatus for producing an error signal when a complex dielectric constant of a process aqueous dielectric fluid differs from a complex dielectric constant of a reference dielectric fluid comprising:
(a) a power source;
(b) a differential capacitance probe coupled to the power source comprising:
(i) a first capacitor probe configured for being immersed in the process aqueous dielectric fluid, and
(ii) a second capacitor probe configured for being immersed in the reference dielectric fluid and isolated from the process aqueous dielectric fluid,
wherein the first capacitor probe and the second capacitor probe are configured in parallel, the first capacitor probe is configured for generating a first current measurement signal, and the second capacitor probe is configured for generating a second current measurement signal;
(c) a device coupled to the differential capacitance probe for receiving the first current measurement signal and the second current measurement signal and producing an error signal when the first current measurement signal differs from the second current measurement signal.
Still another preferred embodiment of the invention comprises a method for producing an error signal when a complex dielectric constant of a process aqueous dielectric fluid differs from a complex dielectric constant of a reference dielectric fluid comprising:
(a) in connection with a system for controlling a process including the process aqueous dielectric fluid having at least one variable parameter, the system comprising:
(i) a first probe configured for being immersed in the process aqueous dielectric fluid, the first probe configured for measuring at least the capacitance of the process aqueous dielectric fluid,
(ii) a second probe configured for being immersed in the reference dielectric fluid wherein the reference dielectric fluid is isolated from the process aqueous dielectric fluid, the second probe configured for measuring at least the capacitance of the reference dielectric fluid,
(iii) a power source, the first probe and the second probe being connected to the power source such that the current provided to the first probe and the second probe is directly proportional and the first probe generates a first measurement signal and the second probe generates a second measurement signal;
(iv) means for detecting the difference between the first measurement signal and the second measurement signal and outputting an error signal, the error signal being used to promotionally alter the variable parameter to modify the process aqueous dielectric fluid, immersing the second probe in the isolated reference dielectric fluid;
(b) immersing the first probe and the isolated second dielectric fluid, having the second probe immersed therein, in the process aqueous dielectric fluid;
(c) causing the power source to provide current to the first probe and the second probe such that the first probe generates the first measurement signal and the second probe generates the second measurement signal such that the means for detecting thereby outputs the error signal when the dielectric constant of the process aqueous dielectric fluid differs from the dielectric constant of the reference dielectric fluid.